Support non-local session store

Product Feedback: Copilot CLI SDK — Sessions are local-only

The session.list RPC only reads from ~/.copilot/ on the local filesystem — there's no cloud-backed API to fetch session history remotely even though we are authenticating via the same PAT across different environments/servers.

This makes it difficult to build production-deployed apps (e.g. Docker containers) that display a user's Copilot CLI sessions. Auth works fine, but session.list always returns 0 results in any non-local environment.

🙏 Feature request: A cloud API endpoint like GET /user/copilot/sessions (similar to how Copilot Chat syncs sessions in VS Code) so session history is accessible remotely as long as the user authenticated via the same PAT